Air quality measuring device

ABSTRACT

The present invention provides a device for measuring air quality. The device includes at least one sensor with a sensor resistance which is converted via signal processing into an output signal, the gas sensing or air content sensing device having its sensor(s) with electrical resistance that is a function of the air quality, such sensors being connected in an astable flipflop circuit arrangement with a first feedback resistor, a second feedback resistor, a capacitor and a comparator--where the first resistor is coupled between the inverting input and output of the comparator, and the second resistor is coupled between the noninverted input and the output of the comparator, and the sensor(s) is/are coupled between a reference potential and the noninverting input of the comparator. The output signal has a period which is related to the sensor resistance.

FIELD OF THE INVENTION

The present invention relates to a device for measuring air quality.

BACKGROUND INFORMATION

In the related art, an analog signal controls a voltage-controlledoscillator (VCO). The voltage-controlled oscillator delivers a digitalsignal whose frequency is a measure of the voltage of the analog signal.

SUMMARY OF THE INVENTION

The present invention provides a trouble-free signal processing systemfor an air quality sensor.

In one embodiment of the device according to the present invention formeasuring air quality, a sensor is provided whose sensor resistance isrelated to air quality. This sensor resistance is part of a signalprocessing system that converts the sensor resistance into an outputsignal with a period which is related to the sensor resistance. Theperiod of the output signal is thus a measure of sensor resistance. Thisfrequency-determined output signal is less sensitive to receivedinterference or conductor-bound interference than would be an analogsignal whose amplitude contains the useful information. Interference dueto temperature effects can also be reduced. Time- andfrequency-dependent output signals can be detected with a higherresolution than analog signals with some microcontrollers. If a binarysignal with a variable period is available as the output signal, simplesignal processing using digital technology is possible. This reduces theneed for expensive analog components.

Voltage-time or voltage-frequency converters may be used for signalprocessing. These circuits, also known as voltage-controlledoscillators, are available as standard integrated circuits or chips.

In another embodiment according to present invention, an astableflipflop arrangement including an RC element and a comparator is used asthe voltage-time converter. According to this embodiment, a capacitor iscoupled between a reference potential and an inverting input of thecomparator, and a first resistor is coupled to the output of thecomparator. A sensor resistance is coupled between the referencepotential and a non-inverting input of the comparator, and a secondresistor is coupled to the output of the comparator. The output signalis picked off at the output of the comparator. This circuit designyields the result that the period of the output signal is a logarithmicfunction of the sensor resistance. The period of this logarithmic curvecorresponds to the sensor resistance/air quality characteristics of theair quality sensors, which are also logarithmic or exponential. Thisyields a constant sensitivity over several decades. The need for analoglog modules, which would also yield a constant sensitivity, is thusreduced. An output signal with a variable period is less susceptible tointerference in comparison with the output signal of an analog logmodule.

The present invention also provides an air quality measuring device foruse in motor vehicles. The ventilation systems are regulated as afunction of the air quality.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a block diagram of a device for measuring airquality.

FIG. 2 illustrates a circuit arrangement of one embodiment of the devicefor measuring air quality.

FIG. 3a illustrates an output signal in accordance with an embodiment ofthe present invention.

FIG. 3b illustrates the output signal in accordance with anotherembodiment of the present invention.

DETAILED DESCRIPTION

An air quality 10 is converted by a sensor 11 into a test signal 12.Signal processing system 13 converts this test signal 12 into an outputsignal 14.

In one embodiment according to the present invention, signal processingsystem 13 includes a comparator 21, at whose non-inverting input asensor resistor R_(s) 17 is connected to a reference potential, and asecond resistor R2 19 is connected to the output of comparator 21. Acapacitor C 18 is connected to a reference potential at the invertinginput of comparator 21, and a resistor R 20 is connected to the outputof comparator 21. Output signal 14 is picked off against the referencepotential at the output of comparator 21.

FIG. 3a illustrates a graph of the output signal 14 as a function oftime. The output signal 14 is characterized by an exponential rise andfall, which is repeated after first period T1. Binary output signal 14shown in FIG. 3b is repeated with a second period T2. Within this secondperiod T2, binary output signal 14 has a value of logical 1corresponding to voltage U during a third period T3.

The device for measuring air quality functions as follows: sensor 11 isto determine air quality 10. Air quality 10 is understood in particularto refer to the composition of the air such as the prevailingconcentration of carbon monoxide or nitrogen oxide. Sensor resistanceR_(s) 17 of sensor 11 changes as a function of the pollutionconcentration. The CO and NO_(x) sensors conventionally used have anexponential resistance/concentration characteristic. A resistive sensormay also be used as sensor 11. All these sensors 11 deliver as testsignal 12 a sensor resistance R_(s) 17 as a function of air quality 10.Since the characteristics have a correspondingly large value range overseveral decades because of their exponential function, logarithmicanalysis is appropriate to obtain a constant sensitivity.

The ventilation system in a motor vehicle, for example, can becontrolled as a function of the air quality. A high pollutionconcentration, such as that occurring in tunnels, for example, wouldtrigger the fan to shut off and/or ventilation valves to close.

Signal processing system 13 converts test signal 12 into an outputsignal 14 with variable periods T, T1, T2, T3. Voltage-time orvoltage-frequency converters, for example, can be used as signalprocessing system 13. For example, a voltage proportional to sensorresistance R_(s) 17 of sensor 11 that is picked off at sensor resistorR_(s) 17 of sensor 11 serves as the input voltage of these converters.The end effect is to achieve an analog-digital conversion.

With a pulse width converter, the voltage to be measured is comparedwith a sawtooth voltage. A dual slope analog-digital converter is alsosuitable for representing an analog voltage as a function of time. Thevoltage-frequency converter receives as an input quantity, the voltageto be converted, and delivers a train of square-wave pulses. Thefrequency of these pulses is proportional to the applied voltage.Voltage-frequency conversion can also performed according to the chargebalance method. The voltage-frequency characteristic with this knownmethod is a straight line.

FIG. 2 illustrates an astable flipflop arrangement, also known as amultivibrator or a relaxation oscillator, which includes an RC elementand a comparator. Sensor resistance R_(s) 17 determines the frequency inthis circuit. At the time when the flipflop shown in FIG. 2 isenergized, the voltage on the capacitor is zero. Comparator 21 suppliesas output signal 14 a positive voltage U, as shown in FIG. 3b. Thisvoltage charges capacitor C 18 across resistor R20. The operating pointin the form of the falling voltage on sensor resistance R_(s) 17 isdefined by the voltage divider, including sensor resistor R_(s) 17 and asecond resistor R2 19, and by output signal 14. When the voltage oncapacitor C 18 reaches this operating point, comparator 21 flips andsupplies an output voltage -U. The charging current of capacitor C 18 isnow flowing in the opposite direction. Capacitor C 18 discharges untilreaching a second operating point at the negative value of the firstoperating point. Output signal 14 of comparator 21 again changespolarity, with positive voltage U being applied at the output. Theprocess described above is then repeated.

With the help of the exponential charging and discharging curve ofcapacitor C 18, periods T, T1, T2, T3 can now be determinedmathematically. ##EQU1## If the quotient 2R_(s) /R2 is much greater than1, equation (1) is simplified as follows: ##EQU2##

This logarithmic relationship between period T and sensor resistanceR_(s) 17 is based on a circuit configuration as illustrated in FIG. 2.An output signal 14 as illustrated in FIG. 3b is realistic for thiscircuit configuration. Second period T2 shown there corresponds toperiod T in equations (1) and (2). The logarithmic relationship betweenperiod T and sensor resistance R_(s) 17 is thus adapted to thecharacteristic of sensor 11 described previously. This permits, inparticular, a constant sensitivity over several decades.

The present invention is explicitly not limited to the embodimentillustrated in FIG. 2. Thus, output signal curves, other than thoseillustrated in FIGS. 3a and 3b, are also contemplated by the presentinvention. Sensor resistance R_(s) 17 may be converted, for example, toan output signal 14 having a first period T1, which increasesexponentially within first period T1 and then drops again. After firstperiod T1, this exponential rise and fall is repeated. In agreement withFIG. 3b, sensor resistance R_(s) 17 may be converted to a period T, T1,T2, T3, with a third period T3 being varied at a constant second periodT2, which represents a basic period. This corresponds to an influence onthe pulse duty factor. In addition, there is the possibility of varyingboth basic period T2 and the pulse duty factor through T3. Suitablemodulation methods are to be used for this purpose.

Standard components are available for determining period T, T1, T2, T3which depends on sensor resistance R_(s) 17. In addition to digitaldetermination, conversion to an analog voltage is also possible. Somemicro-controllers are characterized in that analog voltages are measuredwith 8-bit resolution, for example, but times are measured with 16-bitresolution.

In the circuit configuration illustrated in FIG. 2, the common referencepotential is ground. However, this is not necessarily the case. Forexample, it is quite feasible to bring sensor resistance R_(s) 17 andcapacitor C 18 to a common reference potential, whereas output signal 14is picked off against a second reference potential which matches apotential supplied to comparator 21. In this way, signal adjustments canbe performed in a controlled manner.

I claim:
 1. A air sensing device for measuring air quality,comprising:at least one sensor having a electrical resistance which is afunction of the air quality; and an astable flipflop circuit arrangementincluding a first feedback resistor, a second feedback resistor, acapacitor and a comparator, the first resistor being coupled between aninverting input of the comparator and an output of the comparator, thesecond resistor being coupled between a noninverting input of thecomparator and the output of the comparator, the capacitor being coupledbetween a reference voltage and the inverting input of the comparator,the at least one sensor being coupled between a reference potential andthe noninverting input of the comparator, and the output of thecomparator providing an output signal.
 2. The device according to claim1, wherein a resistance of the second resistor is much smaller than theresistance of the at least one sensor.
 3. The device according to claim1, wherein the resistance of the at least one sensor is a resistiveresistance.
 4. The device according to claim 1, wherein the outputsignal has a period that is a logarithmic function of the resistance ofthe at least one sensor.
 5. An air quality measuring device with airsensing capability for controlling a ventilation system in a motorvehicle, comprising:at least one sensor having a electrical resistancewhich is a function of the air quality; an astable flipflop circuitarrangement including a first feedback resistor, a second feedbackresistor, a capacitor and a comparator, the first resistor being coupledbetween an inverting input of the comparator and an output of thecomparator, the second resistor being coupled between a noninvertinginput of the comparator and the output of the comparator, the capacitorbeing coupled between a reference voltage and the inverting input of thecomparator, the at least one sensor being coupled between a referencepotential and the noninverting input of the comparator, the output ofthe comparator providing an output signal; and the ventilation systemcontrolled by a control signal which is a function of the output signal.